A reverse osmosis system, for example, serves the purpose of gaining drinking water from salt water or waste water, in the following called “feed water”. For this purpose, the high-pressure pump brings the feed water to a relatively high pressure, for example 80 bar or more, after which the feed water is supplied to the membrane unit. The membrane unit comprises a semi permeable membrane retaining the impurities or the salt of the feed water, only letting the cleaned water, called permeate, pass. The salt and dirt concentration in the remaining feed water then increases. The feed water with the increased concentration, also called concentrate, is discharged from the membrane unit through the concentrate outlet. However, this concentrate is still at a relatively high pressure of, for example, 60 to 70 bar, so that there is a wish to regain the energy content of the concentrate. However, the invention is not limited to the use of water as fluid.
It is therefore known to connect the concentrate outlet to a pressure exchanger. On its concentrate side, the pressure exchanger is supplied with the concentrate with the increased pressure. On the other side, the supply side, the feed water is supplied, whose pressure is increased by the concentrate. Pressure exchangers are, for example, described in DE 37 81 148 T2, U.S. Pat. No. 5,338,158, U.S. Pat. No. 5,988,993, WO 99/17028 A1, U.S. Pat. No. 6,540,487 B2 or U.S. Pat. No. 7,214,315 B2. The latter document also mentions the use in a reverse osmosis process.
The pressure exchanger or pressure converter can, however, usually not transfer the whole pressure of the concentrate to the feed water. Also, usually the membrane unit has a certain pressure loss between the inlet and the concentrate outlet. In order to bring the share of the feed water that has been brought to a higher pressure by means of the pressure exchanger up to the pressure required by the membrane unit, a booster pump is therefore required. This booster pump has to be driven, meaning that additional energy is required. The control of it is problematic. If the booster pump transports too much fluid, it may happen that it does not only transport feed water but also concentrate, so that the concentrate concentration on the inlet side of the membrane increases, which again causes a reduced efficiency. This phenomenon is called “mixing”. An increased salt content increases the energy consumption. Many systems therefore use one or more flow meters to prevent a mixing of the fluids.